1. Field of the Invention
The present invention relates in general to an apparatus for controlling an engine brake force of a vehicle equipped with an automatic transmission, and more particularly to such an engine brake force control apparatus capable of increasing the engine brake force when the vehicle is accelerated on a downhill with the accelerator pedal released, and/or controlling the engine brake force when the brake pedal is depressed after the accelerator pedal is released during running on a downhill.
2. Discussion of the Related Art
Generally, an automatic transmission of a motor vehicle is controlled so that the speed ratio of the transmission is changed in steps or continuously on the basis of the operating amount of the accelerator pedal and the opening of the throttle valve of the engine. An example of a shift pattern data map for shifting the automatic transmission having four forward-drive gear positions is illustrated in FIG. 9. This data map indicates shift-up and shift-down boundaries which are relationships between the operating amount of the accelerator pedal and the throttle opening. The solid lines in FIG. 9 show the shift-up boundaries, while the broken lines show the shift-down boundaries. According to this shift pattern data map, the transmission is shifted up depending upon the vehicle running speed, even when the operating amount of the accelerator pedal is zero, that is, when the accelerator pedal is in the released position. Therefore, the transmission may be shifted up when the driver releases the accelerator pedal during running of the vehicle on a downhill and the vehicle speed rises without a sufficient engine brake force applied. This shift-up action of the transmission further reduces the engine brake force, and tends to accelerate the vehicle. To solve this problem, the assignee of the present application proposed an engine brake force control apparatus as disclosed in Japanese Patent Application No. 3-352773 which has not been laid open at the time the present invention was made. In the proposed apparatus, the actual vehicle speed at the time the accelerator pedal is released by the driver to avoid acceleration of the vehicle is used as the desired vehicle speed, and the throttle opening is reduced to control the engine brake force such that the actual vehicle speed will not exceed the desired speed.
It was found, however, that the mere controlling the engine brake force for the vehicle speed not to exceed the desired speed may suffer from reduction of the vehicle speed unexpected to the vehicle driver, due to an excessive engine brake force, when the gradient of the downhill road is reduced. To avoid this phenomenon, the engine brake force should be controlled by increasing or decreasing the throttle opening so that the actual vehicle speed is made equal to the desired speed.
In the proposed control apparatus described above, the throttle opening is controlled to maintain the actual vehicle speed at or near the desired speed, even when the road gradient is changed from a downhill to an uphill. Usually, the driver expects some deceleration of the vehicle after the vehicle begins to run on the uphill. However, the above arrangement maintains the vehicle speed at the constant desired level, unexpectedly to the driver. That is, the driver expects that the acceleration of the vehicle is prevented or restricted by an engine brake force applied thereto when the accelerator pedal is released during running on a downhill, and also expects that the vehicle is decelerated on an uphill, but does not expect constant-speed running of the vehicle during the uphill running as achieved by an automatic cruising device.
The proposed control apparatus also suffers from a problem that when the driver depresses the brake pedal to further decelerate the vehicle after the accelerator pedal is released. In this case, the engine brake force will not be increased after the actual vehicle speed is lowered down to the desired speed, whereas the driver's desire to decelerate the vehicle is still large. Accordingly, the driver has to rely on the brake force produced by the brake pedal depressed by the driver. Thus, the initial depression of the brake pedal eventually requires an increased depressing force acting on the brake pedal, due to an insufficient engine braking effect during running of the vehicle with the accelerator pedal released. In some case, the engine produces a forward drive force, with no engine brake applied to the vehicle.
The proposed control apparatus further suffers from a problem that when the driver releases the brake pedal after the vehicle speed is lowered to a level desired by the driver, the engine brake force may not be sufficient, result in the vehicle speed rising back to the desired speed at the time the accelerator pedal was released. For example, when the driver depresses the brake pedal to decelerate the vehicle during running on a straight downhill and releases the brake pedal shortly before the lower end of the downhill which precedes a curve, the vehicle will be accelerate during the subsequent running along the curve, requiring the driver to again depress the brake pedal. Thus, the proposed apparatus is not satisfactory in terms of the ease of control of the vehicle equipped with an automatic transmission.